The invention relates to frequency compensation of electronic circuits and, in particular, to circuits having compensating capacitors.
Stability of circuit feedback is improved by compensating to increase phase margin. A well known technique for improving phase margin takes advantage of Miller Effect, by adding a Miller-compensating capacitor in parallel with a gain stage, e.g., the output stage of a two stage amplifier circuit. Such a configuration results in the well-known and desirable phenomenon of pole splitting, which advantageously multiplies the effective capacitance of the physical capacitor employed in the circuit. Background on compensation of amplifier circuits using Miller-compensating capacitor can be found in Paul R. Gray and Robert G. Meyer, Analysis and Design of Analog Integrated Circuits, Third Ed., John Wiley & Sons, Inc., New York, 1993, Ch. 9, especially pp. 607-623.
U.S. Pat. No. 6,084,475 discloses a compensated amplifier amplifying an input signal applied to an input node to provide an output signal at an amplifier output node. As shown in FIG. 1, the compensated amplifier 100 comprises a first amplifier stage 110, a second amplifier stage 120, a third amplifier stage 130, and a capacitor C. The first amplifier stage 110 has an internal node as an input thereto and a first stage output node. The second amplifier stage 120 has the amplifier input node as an input thereto and a second stage output node. The third amplifier stage 130 has a third stage input node coupled to the first stage output node and the second stage output node and provides the output signal at the amplifier output node. The capacitor C is coupled between the amplifier output node and the internal node. By connecting the capacitor C in such a way, feedback current through the capacitor C is amplified by the second amplifier stage 120 before reaching a high impedance node. The resulting effective load capacitance at the high impedance node is also amplified. A main signal path of the compensated amplifier is shown as a dashed arrow in FIG. 1. The main signal is received by the second amplifier stage 120 and amplified by the third amplifier stage 130. Nevertheless, a product of gain and bandwidth of the compensated amplifier decrease with amplification of the capacitance.